Molecular characterization of Flavobacterium columnare isolated from a natural outbreak of columnaris disease in farmed fish, Catla catla from India.

Flavobacterium columnare is currently one of the important bacterial pathogens causing columnaris disease in several farmed fish species across diverse geographies. A presumptive columnaris disease outbreak in farmed catla, Catla catla (Hamilton), was investigated with the aim of isolating and identifying the causative pathogen. F. columnare (strain RDC-1) was isolated from gills of infected fish and identified by conventional biochemical methods, and through species specific polymerase chain reaction and sequencing of the 16S rDNA for molecular identification. Strain RDC-1 belonged to genomovar II with 99% similarity to available 16S rDNA sequences of F. columnare, and also shared 70% DNA-DNA relatedness with known strains of F. columnare. Bath immersion studies of RDC-1 showed development of columnaris disease in catla fingerlings within 7 days, with a cumulative mortality of 83.3%. This is the first molecular confirmation of Flavobacterium columnare as a fish pathogen of farmed Catla catla in India.